One application of alternative energy sources is the delivery of power to an alternating-current (AC) utility grid. In such applications, an inverter (i.e., a DC-AC power conditioner) is required in order to convert the DC power delivered by the alternative energy source into sinusoidal alternating-current (AC) power at the grid frequency. Certain inverters (e.g., those used by residential customers or small businesses) convert the DC power delivered by the alternative energy source into single-phase AC power and deliver a sinusoidal current to the AC grid at the grid frequency. One figure of merit for such inverters is the utilization ratio, which is the percentage of available power that the inverter can extract from an energy source. Ideally, an inverter will achieve a utilization ratio of 100%.
A basic electrical property of a single-phase AC power system is that the energy flow includes both an average power portion that delivers useful energy from the energy source to the load and a double-frequency portion that flows back and forth between the load and the source:p(t)=Po+Po*cos(2ωt+φ)
In applications involving inverters, the double-frequency portion (Po*cos(2ωt+φ)) represents undesirable ripple power that, if reflected back into the DC power source, may compromise performance of the source. Such concern is particularly worrisome for photovoltaic cells.
Photovoltaic cells have a single operating point at which the values of the current and voltage of the cell result in a maximum power output. This “maximum power point” (“MPP”) is a function of environmental variables, including light intensity and temperature. Inverters for photovoltaic systems typically comprise some form of maximum power point tracking (“MPPT”) as a means of finding and tracking the maximum power point (“MPP”) and adjusting the inverter to exploit the full power capacity of the cell at the MPP. Extracting maximum power from a photovoltaic cell typically requires that the cell operate continuously at its MPP; fluctuations in power demand, caused, for example, by double-frequency ripple power being reflected back into the cell, will compromise the ability of the inverter to deliver the cell's maximum power. Thus, to maximize the utilization ratio, an inverter for photovoltaic energy systems should draw only the average power portion of the energy flow from the photovoltaic cells at the inverter input. Such inverters should therefore comprise means to manage the double-frequency ripple power without reflecting the ripple power back into the source.
To manage double-frequency ripple power, energy needs to be stored and delivered at twice the AC frequency. One way to manage the double-frequency ripple power is to use passive filtering in the form of capacitance across a DC bus of the inverter. However, passive filtering alone may require a relatively large capacitance value to filter the double-frequency power, since the double-frequency energy exchange needs to be supported by the capacitor without imposing significant voltage ripple on the DC bus. Controlling the duty cycle of a regulator that is connected between the source and the DC bus may enable further attenuation, over and above that provided by the bus capacitor alone, in the amount of the double-frequency ripple power that is reflected back into the source.
Another way to manage double-frequency ripple power is to use an active filter circuit that supplies the double-frequency ripple power by means of a capacitor internal to the active filter. Whereas the passive filtering approach requires a relatively large filter capacitor, the internal capacitor in an active filter may be made relatively smaller, since it is only required to store and deliver the double-frequency ripple power and is not required to support the DC bus voltage. Because the active filter “isolates” the internal capacitor from the DC bus, the voltage variation across the internal capacitor can be relatively large and the value of the capacitor may be made relatively small.